I've yet to move beyond the mammal realm, however...
Scardykat85's got a couple of fish partials (I'm in love with the angler fish):
Koi-JoyFreezePopMixedCandy's made a porpoise and an orca:
http://mixedcandymascots.com/gallery/v/aquatic+animals/Unfortunately, I didn't see any back shots of the fish, though they seem to be made of fleece and the tails probably have a belt attachment, given their sizes. MixedCandy's suits look like they're attached straight to the body suit and, I assume that there's more of a harness system to them since they're on the larger side. They look like they used a short-pile seal fur or a velvety material, but I can't be sure.
Oh, and of course, Lacy's made a spotted seal:
http://lacy.timduru.org/gallery.php?a=folder&id=98I also saw a massive orca tail on furbid a while ago. It was made with short-pile fur and appeared to be designed to be worn with two belts or a harness of some kind.